This invention relates to aqueous-based photosensitive thermally developable emulsions and photothermographic materials that include silver halide core-shell grains containing high amounts of silver iodide. It also relates to methods of imaging the photothermographic materials.
Silver-containing photothermographic imaging materials that are developed with heat and without liquid development have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, visible, ultraviolet, or infrared radiation) and developed by the use of thermal energy. These materials, also known as xe2x80x9cdry silverxe2x80x9d materials, generally comprise a support having coated thereon: (a) a photosensitive catalyst (such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires close physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag0)n, also known as silver specks, clusters, nuclei, or latent image, arc generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [Kilosterboer, Imaging Processes and Materials (Neblette""s Eighth Edition), Sturge, Walworth and Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pp. 279-291, 1989]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide, have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].
The photosensitive silver halide may be made xe2x80x9cin situ,xe2x80x9d for example, by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be co-precipitated [see Usanov et al., J. Imag. Sci. Tech. 40, 104 (1996)]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Usanov et al., International Conference on Imaging Science, 7-11 Sept. 1998).
The silver halide may also be xe2x80x9cpreformedxe2x80x9d and prepared by an xe2x80x9cex situxe2x80x9d process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to, and be present during, the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example, U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.
The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as xe2x80x9cfatty acidsxe2x80x9d or xe2x80x9cfatty carboxylic acidsxe2x80x9d. Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a xe2x80x9cdeveloper,xe2x80x9d may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent for silver ion. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
The various distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Most common photothermographic materials are prepared using organic solvents for layer formulation and coating, and are therefore often identified as xe2x80x9csolvent-basedxe2x80x9d or xe2x80x9cnon-aqueousxe2x80x9d materials. The various chemical components required for such materials are generally soluble in the organic solvents and insoluble in water.
However, photothermographic materials that can be formulated and coated out of water (xe2x80x9caqueous-basedxe2x80x9d materials) would have a number of manufacturing, environmental, and cost advantages. Use of the same chemical components that are present in solvent-based materials is not always possible in aqueous environments without the use of expensive or tedious solubilizing or dispersing techniques. The water-insoluble chemical components tend to precipitate and cause variability in photosensitive response and coating defects when used in aqueous formulations even with adequate dispersion.
One major effort in the development of aqueous-based photothermographic materials has been to increase image density (Dmax). One way to do this is to increase the amount of silver in the imaging environment (or emulsion). However, increasing the silver coverage may increase image xe2x80x9cprint-outxe2x80x9d or an increase in Dmin over time. This effect diminishes the usefulness and accuracy of the image. Very-high surface iodide containing silver halide emulsions have not been of much interest in photographic films because they are difficult to chemically and spectrally sensitize and have relatively slow developing and fixing speeds. Making emulsions with core-shell structures in which the shell has a lower iodide content than the core has reduced or eliminated these problems. Core-shell emulsions are described for example in U.S. Pat. No. 4,728,602 (Shibahara et al.).
The use of core-shell emulsions in heat-developing photographic film is described in U.S. Pat. No. 5,064,753 (Sohei et al.). The silver halide itself is the primary component reduced to silver metal during development and this material is primarily used for color applications. The print-out properties of the preferred formulations are not addressed. High-surface-iodide grains are said to cause enhanced thermal fog.
In photothermographic materials relaying on a non-photosensitive reducible source of silver and coated using a non-aqueous binder, the light-sensitive silver halide core-shell emulsion preferably has a total iodide level of less than 10 mole % as described in U.S. Pat. No. 5,434,043 (Zou et al.) and less than 4 mole % as described U.S. Pat. No. 5,382,504 (Shor et al.).
There is an advantage in cost and to the environment in coating photothermographic materials as an aqueous-based (hydrophilic) system. However, the use of hydrophilic binders may result in rapid high humidity print-out of processed film. Hence, there is a need for improved aqueous-based (hydrophilic) photothermographic materials that exhibit desired high Dmax while image xe2x80x9cprint-outxe2x80x9d is reduced.
The present invention provides a thermally developable emulsion comprising:
a) core-shell grains of a photosensitive silver halide,
b) a non-photosensitive source of reducible silver ions,
c) a hydrophilic binder,
d) a reducing agent composition for the reducible silver ions, and
e) a cyclic imide, benzoxazine dione, benzthiazine dione, triazole thione, quinazoline dione, or phthalazinone as a development promoter,
wherein predominantly all of the core-shell photosensitive silver halide grains comprise at least 20 mol % iodide based on total silver, an amount of iodide in the core of the grains that can be up to the iodide saturation limit in silver iodobromide, and an amount of iodide in the shell of the grains that is at least 2 mol % less than the amount of iodide present in the core, and
further provided that the total amount of silver in the shell is from about 10 to about 80 mol % of total silver in the grains.
This invention also provides a photothermographic material comprising a support having thereon at least one imaging layer comprising a hydrophilic binder, and having in reactive association:
a) core-shell grains of a photosensitive silver halide,
b) a non-photosensitive source of reducible silver ions,
c) a reducing agent composition for the reducible silver ions, and
d) a cyclic imide, benzoxazine dione, benzthiazine dione, triazole thione, quinazoline dione, or phthalazinone as a development promoter,
wherein predominantly all of the core-shell photosensitive silver halide grains comprise at least 20 mol % iodide based on total silver, an amount of iodide in the core of the grains that can be up to the iodide saturation limit in silver iodobromide, and an amount of iodide in the shell of the grains that is at least 2 mol % less than the amount of iodide present in the core, and
further provided that the total amount of silver in the shell is from about 10 to about 80 mol % of total silver in the grains.
In preferred embodiments the photothermographic materials of this invention comprise a transparent support having thereon an aqueous-based imaging layer comprising gelatin or a gelatin derivative as binder,
an aqueous-based surface protective overcoat over the imaging layer, and an aqueous-based antihalation layer on the backside of the support, and
the imaging layer having in reactive association:
a) core-shell grains of photosensitive silver iodobromide,
b) a non-photosensitive source of reducible silver ions that comprises one or more silver carboxylates provided as an aqueous nanoparticulate dispersion, at least one of which silver carboxylates is silver behenate,
c) a reducing agent composition for the reducible silver ions that includes one or more hindered phenols,
d) one or more antifoggants or spectral sensitizing dyes, and
e) succinimide, 2H-1,3-benzoxazine-2,4-(3H)-dione, or phthalazinone as a development promoter,
wherein said cores of the grains comprise iodide in an amount of from about 25 to about 37 mol %, based on total silver in the grain cores, the shells of the grains comprise an amount of iodide that is at least 10 mol % less than the amount of iodide present in the core, and
further provided that the total amount of silver in the shell is from about 10 to about 40 mol % of total silver in the grains, and the amount of total silver from silver halide is from about 0.02 to about 1 g/m2.
In addition, this invention provides a method of forming a visible image comprising:
A) imagewise exposing the photothermographic material of this invention to electromagnetic radiation at a wavelength greater than 400 nm to form a latent image,
B) simultaneously or sequentially, heating the exposed photothermographic material to develop the latent image into a visible image.
In some embodiments, the photothermographic material comprises a transparent support, and the image-forming method further comprises:
C) positioning the exposed and heat-developed photothermographic material having the visible image therein between a source of imaging radiation and an imageable material that is sensitive to the imaging radiation, and
D) thereafter exposing the imageable material to the imaging radiation through the visible image in the exposed and heat-developed photothermographic material to provide a visible image in the imageable material grains.
The photosensitive and thermally sensitive emulsions and materials of this invention provide better incubation keeping (that is, reduced incubation fog), acceptable image density (Dmax) and the resulting images exhibit reduced image xe2x80x9cprintoutxe2x80x9d. These advantages provide latitude in how much silver is used in the emulsion. Other sensitometric properties are maintained at acceptable values. These advantages are achieved by using core-shell photosensitive silver halide grains that include a higher than normal amount of iodide and a specific amount of iodide in the shells in relation to the amount of iodide in the cores of the grains.